172 THE BLOOD AND ITS CIRCULATION. 



crease, and the tension of the blood also. Now, what takes 

 place is precisely the contrary ; Cl. Bernard has shown by 

 the aid of the differential manometer, that the tension is 

 increased in the first instance, and diminished in the second 

 (Legros). 



But how does the great sympathetic nerve act? How 

 does it happen that (during the condition of inactivity) 

 it keeps the vascular coats in a continued state of contrac- 

 tion ? How is it that, at certain moments, by means of 

 reflex actions, this nerve causes nearly similar phenomena to 

 those which it exhibits when cut ; such as dilatation of the 

 vessels, and greater afflux of blood in certain parts of the 

 organism (sudden redness of the face, turgescence of the erec- 

 tile tissues, hyperaemia, more abundant secretion of the 

 glands, etc.) ? 



In replying to the first question a constant state of excita- 

 tion of the vaso-motor nerves is generally admitted : this 

 being due to a continuous reflex action originating in the 

 sensitive nerves of the arteries (Audiffrent) in other sensi- 

 tive parts; thus the muscular tonus has been looked upon 

 as a reflex influence : according to Brondgeest, the tonus 

 may be made to cease instantly by section of the sensory 

 nerves proceeding from any part which may be in a tonic con- 

 dition. According to other physiologists, the constant excita- 

 tion of the vaso-motor centre is produced by the presence of 

 carbonic acid in the blood. If animals be poisoned by means 

 of this acid, all the small arteries will be found in a con- 

 tracted state (Thiry). 



The second question is still more difficult to answer. It 

 has been clearly demonstrated that repeated excitations pro- 

 duce dilatation of the vessels by reflex action ; if the ear 

 of a rabbit be cut off, and the sciatic nerve excited, we see 

 that the blood flows in much greater abundance through the 

 vessels which have been cut. Again, there are centrifugal 

 nerves, irritation of which causes instant dilatation of the 

 vessels; thus the chorda tympani, if irritated, produces 

 severe hyperaemia, and consequently, abundant secretion in 

 the sub-maxillary gland. 



It is difficult to allow the existence of nerves which directly 

 paralyze the muscular elements of the arterial tunics; for 

 instance, the chorda tympani, which is a branch of the 

 facial nerve, reminds us rather of those nerves which, by 

 their influence upon others, cause all action to cease in the 

 latter, by a sort of nervous interference, as the intervention 



